The world's use of fossil fuels has grown exponentially over the past several decades. With this growth, the oil and gas industry has broadened the search for new oil and gas reserves to meet the ever growing consumer demand. The search for new oil and gas reserves now includes areas previously unimaginable for exploration. The need to produce oil and gas from these new regions has presented a new set of problems related to the design and validation of equipment used in the production of the newly discovered oil and gas reserves.
Some of the new areas which are producing substantial oil and gas reserves are located beneath the ocean. In the subsea environment, new problems associated with flow line and production equipment have produced a new class of equipment design problems which are sometimes referred to as “Hydrogen Induced Stress Cracking” (HISC). In general, HISC problems are created by two environmental factors, specifically the availability of ionic hydrogen at the surface of chromium alloyed steel constructed subsea equipment due to such equipments' immersion in an aqueous solution.
The result of such HISC related problems is manifested by a weakening of the alloyed steel components and structures. Subsequent component and/or structural failure can occur leading to safety issues, environmental damage by contamination to the surrounding subsea location and high repair costs based on equipment location. Analysis of failed subsea systems has indicated that consideration of HISC factors should be included in the overall design process associated with subsea systems which are made of certain materials (e.g. Duplex and Super Duplex Stainless Steel) used in the acquisition and recovery of subsea reserves of oil and gas.
Current methods for analyzing and designing subsea oil and gas production systems, while capable of allowing for HISC considerations, require many hours of computer computational time to complete an analysis of one set of conditions. For example, in a first design/evaluation step, a one-dimensional frame model is developed with center lines representing the axis of the piping system. After the frame model is complete, the axial lines are replaced by finite pipe elements. The pipe elements are able to simulate all various types of operating and non-operating conditions and allow the assessment of ASME requirements (i.e. ASME B31.8).
In a second step, the pipe elements are partially or fully replaced by shell elements. The number of elements replaced is dependent on the sections of the design under review. Like the pipe elements, the shell elements are able to simulate both operating and non-operating conditions. However a significant feature of shell elements in the design process is that the shell elements allow for the prediction of local stresses and therefore the assessment of linear HISC (i.e. DNV RP-F112). However the many sets of load conditions and the associated sets of computational runs associated with processing the shell elements in this second step can be prohibitively expensive both in terms of time and computing requirements.
In a third step, elements identified as not compliant with required conditions based on the linear analysis of step two are replaced with three-dimensional sub-models. An analysis is performed with elasto-plastic material properties allowing the assessment of non-linear HISC conditions. The result of the three-dimensional sub-model analysis allows for the prediction of local strains on the analyzed elements.
Accordingly, market pressure is demanding a method for designing subsea oil and gas equipment capable of withstanding the rigors of the subsea environment without the prohibitively expensive costs, in terms of analysis time and computational requirements, of existing techniques.